1. Field of the Invention
The invention relates to permeable membrane processes and systems for air separation. More particularly, it relates to improved permeable membrane processes and systems for the production of dry nitrogen product gas from feed air.
2. Description of the Prior Art
Permeable membranes capable of selectively permeating oxygen from air are a convenient and desirable means for separating air and recovering nitrogen product gas. Such product gas, however, is generally accompanied by a significant amount of moisture. For some applications, a dry, nearly oxygen-free nitrogen product gas is required. In small to moderate size applications, such purity levels are presently most economically achieved by initially producing nitrogen at about 98% purity by means of membrane separation of air, and then scavenging the remaining oxygen in a post cleanup technique. The most readily available oxygen post cleanup approach involves the use of a deoxygenation system, referred to as a "deoxo system", for converting oxygen to water by combining the oxygen with hydrogen over a noble metal catalyst. The deoxo reaction generates a significant amount of heat, typically raising the exit gas temperature approximately 300.degree. F. per 1% of oxygen removed. The resulting nitrogen product contains less than about 5 ppm oxygen, but substantial quantities of water, e.g. 30-40,000 ppm, and residual hydrogen. In many applications, it is desirable to remove such moisture content, either to prevent condensation and corrosion in plant piping and instrumentation, or because its presence is incompatible with the intended end use of the nitrogen product gas. An aftercooler, moisture separator and adsorptive dryer are generally utilized for this purpose. If an adiabatic pressure swing adsorption (PSA) system and process are used for such drying purposes, a significant fraction of the dry nitrogen product gas may be used as purge gas for the PSA operation. Typically, such a PSA dryer might require a dry purge flow equal to at least 15% of the total product flow in order to achieve a desirable pressure dew point (PDP) of -40.degree. F. In conventional post cleanup applications, a high temperature regeneration cycle, in which the portion of nitrogen product gas used for purge purposes is heated prior to passage to the dryer, is preferred because of the lower purge gas flow requirements of such high temperature operation, typically less than half that required for the adiabatic PSA cycle. This purge flow differential may become even larger if a very dry (-100.degree. F. PDP) product gas is desired. A less common alternative involves the use of a heated, ambient air purge followed by a dry product gas cooling purge to further improve cycle recovery, but at the expense of additional thermal energy expenditure.
Using dry product gas as purge, adsorbent energy requirements may be on the order of 8.times.10.sup.-5 KW per SCFH of product nitrogen. By comparison, the use of wet purge gas increases such energy requirements to approximately 2.5.times.10.sup.-4 KW per SCFH. Thus, a trade-off will be seen to exist between dryer recovery and thermal energy requirements.
Because of the inherent simplicity of permeable membrane systems, there is a strong incentive and desire in the art to employ membrane systems for all types of air separation operations, including those in which dry, high purity nitrogen product gas is required. For such dry nitrogen applications, it is desired to obtain the benefits achieved in the higher temperature dryer regeneration referred to above, while minimizing the energy requirements associated therewith.
It is an object of the invention, therefore, to provide an improved process and system for the production of dry, high purity nitrogen.
It is another object of the invention to provide a process and system for dry, high purity nitrogen production employing high temperature dryer regeneration without the high thermal energy costs associated therewith.
It is a further objective to provide a dry, high purity nitrogen process and system utilizing high temperature dryer regeneration and minimizing thermal energy, product nitrogen recovery and capital cost requirements.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.